An ink-jet printer is known which uses an ink-jet recording system that jets a liquid ink droplet through a nozzle of the ink-jet head onto a recording medium such as a printing paper, thereby recording characters, image and the like on the medium.
The ink-jet printer using the foregoing system has advantages as follows:
(1) the mechanism, construction, and printing process of the system are comparably simple, and no noise is emitted in printing;
(2) being capable of discharging a minimal ink droplet, high resolution printing is possible;
(3) arranging plural color inks facilitates color printing;
(4) the system does not consume much power; and
(5) the price is comparably low.
Owing to these advantages, the ink-jet printer using this system has been spreading rapidly as a printer for not only personal computers but also for various office automation apparatus.
Of the ink-jet printer, the so-called drop-on-demand (DOD) type ink-jet printer, which discharges ink droplets only when a printing instruction is issued to minimize the ink consumption, is the most popular.
As major techniques to discharge ink by this DOD type ink-jet printer, there are the piezoelectric system that applies a pressure to the ink chamber by a force induced by the piezoelectric actuator as an electromechanical transducer and the bubble system that utilizes an expansion pressure of bubbles produced by an instantaneous vaporization of ink by an electric heater.
As a conventional example, a circuit for driving an ink-jet head using the piezoelectric actuator and a method of driving the same will be described with reference to FIG. 14 and FIG. 15.
In the circuit for driving an ink-jet head shown in FIG. 14, the emitter of an NPN transistor Q2 and one terminal of a piezoelectric actuator PZ are connected to the ground terminal GND as the earth potential.
A power source voltage VH for the ink-jet head is applied to the emitter of a PNP transistor Q1, one terminal of a second resistor R2, and one terminal of a third resistor R3.
A control signal S for driving the piezoelectric actuator PZ enters input terminals of a first inverter U1 of the open collector type and a second inverter U2. The output of the first inverter U1 is given through a first resistor R1 to the other terminal of the second resistor R2 and the base of the PNP transistor Q1.
The output of the second inverter U2 is given to the other terminal of the third resistor R3 and the base of the NPN transistor Q2. The collector of the NPN transistor Q2 is connected through a fifth resistor R5 to one terminal of a fourth resistor R4 and to the other terminal of the piezoelectric actuator PZ. The other terminal of the fourth resistor R4 is connected to the collector of the PNP transistor Q1.
The foregoing driving circuit drives the ink-jet head as follows.
Applying a pulse waveform voltage to the piezoelectric actuator PZ deforms a part of a wall in an ink chamber to increase the volume of the ink chamber, and ink is supplied in the increased space inside the ink chamber. And, stopping the voltage supply to the piezoelectric actuator PZ, or applying a pulse waveform voltage of the reverse polarity against the foregoing pulse waveform deforms the part of the wall in the ink chamber in the reverse direction to decrease the volume of the ink chamber, thus discharging ink droplets through the nozzle.
FIG. 15 illustrates waveforms of the control signal S, a driving voltage signal VCp applied to the piezoelectric actuator PZ, and a displacement X of the piezoelectric actuator in the conventional circuit for driving an ink-jet head shown in FIG. 14.
In FIG. 15, an initial period T0, a charge period T1 having a pulse change, and a discharge period T2 constitute one printing cycle.
During the initial period T0, the control signal S is in low level, and the output of the first inverter U1 and the second inverter U2 shown in FIG. 14 are in high impedance.
When the output of the first inverter U1 and the second inverter U2 are in high impedance, each of the bases of the PNP transistor Q1 and the NPN transistor Q2 is biased by the power source voltage VH through the second resistor R2 and the third resistor R3.
Accordingly, the PNP transistor Q1 is nonconductive, and the NPN transistor Q2 is conductive, and the piezoelectric actuator PZ is discharged through the fifth resistor R5, so that the driving voltage signal VCp is at the ground potential, namely, zero volt.
When entering into the charge period T1, the control signal S rapidly goes up to the high level, which turns the outputs of the first inverter U1 and the second inverter U2 into low level, thus turning the NPN transistor Q2 nonconductive and the PNP transistor Q1 conductive. Consequently, the piezoelectric actuator PZ is charged by the power source voltage VH through the fourth resistor R4.
Accordingly, the drive voltage signal VCp goes up toward the power source voltage VH in accordance with a time constant by a product of a resistance of the fourth resistor R4 and an equivalent capacitance Cp of the piezoelectric actuator PZ. This voltage VCp charges the piezoelectric actuator PZ to fill ink into the ink chamber.
When entering into the discharge period T2, the control signal S rapidly goes down to low level, the output of the first inverter Ul and the second inverter U2 go back again to high impedance. Thereby, the PNP transistor Q1 turns nonconductive, the NPN transistor Q2 turns conductive. And then, the drive voltage signal VCp goes down toward the ground potential in accordance with a time constant by a product of a resistance of the fifth resistor R5 and an equivalent capacitance Cp of the piezoelectric actuator PZ, thereby discharging the piezoelectric actuator PZ to discharge ink through the ink chamber.
The rapid rise during the charge period T1 and the rapid fall during the discharge period T2 will generate free vibrations of the piezoelectric actuator PZ and the ink in the ink chamber by the intrinsic frequencies thereof and the free vibrations attenuate gently, which is illustrated in the displacement X of the piezoelectric actuator PZ in FIG. 15 as an example.
When the foregoing ink jet recording system is applied for printing characters and graphics and the like in which the density of dots on the recording media is maximum and constant, namely, when it is applied for printing such characters and graphics in general documents and reports, the recording system can display the full advantages.
However in recent years, picture images in which the intermediate density gradation is required to be presented in full colors continuously or in a step form, for example, three-dimensional picture images with shadows and photographic images and the like have frequently been brought into computer images or office automation equipment, and accordingly, these picture images are strongly desired to be printed in a high quality.
Several techniques have been put forward to display the intermediate density gradation, and one of the most popular techniques of the ink-jet recording system is the so-called dither or density pattern method, which displays one pixel of a printing picture image by means of plural dot groups.
Although the density pattern method varies the black dot number in one pixel of a picture image in a step form in accordance with the gradation, and devises the configuration of the dot pattern to display a pseudo gradation, there is a problem that the method displays one pixel of a printing image by means of plural dot groups, and therefore, the resolution of the printing image will remarkably deteriorate.
That is, to obtain a fine gradation in this density pattern method accompanies an increase of the dot number in one pixel, resulting in an increase of one pixel in the picture image, and the quality of a printing image is deteriorated even if a high resolution printer is used.
To obtain a high resolution picture quality, on the contrary, will result in a problem that a powerfully appealing picture with sufficient gradation cannot be acquired.
On the other hand, the so-called area gradation method has been proposed which directly controls areas of individual dots on recording media to vary the density. The multi-droplet method, which is an area gradation method, constitutes one dot with a set of plural fine ink droplets continuously discharged, controls a volume of ink droplets in accordance with the number of fine ink droplets, and thereby, varies the area of the dot on the recording media to add a gradation to the density.
However, this multi-droplet method needs to put plural fine ink droplets continuously discharged on a position that can be regarded as one picture element on a recording medium so as to form a picture element. Therefore, in a printer construction in which a recording head and a recording medium move continuously and relatively, the speed of response to discharge fine ink droplets has to be increased so as not to fluctuate with time positions on the recording medium at which ink droplets are put in order to acquire a high quality picture image.
It is regarded that the speed of response to discharge ink droplets in the multi-droplet method has to be 10 to 20 times higher than the speed of response of a general ink-jet head. Particularly, in the ink-jet head of the piezoelectric system, free vibrations are generated in the piezoelectric actuator and ink inside the ink chamber, and an discharging motion and the subsequent ink supply are repeated before the free vibrations attenuate.
Accordingly, ink droplets are split and atomized, making it difficult to form stable ink droplets and resulting in a problem that the speed of response cannot be increased.
Further, in the multi-droplet method, as ink droplets combine randomly in flight of fine ink droplets or owing to a flight direction thereof, the characteristics of ink droplets change influencing on the shape of a picture element and the stability when ink droplets are put on a recording medium, posing a problem in the stable discharge of continuous fine ink droplets.
Further, there is another method to display the density gradation, in which by controlling the driving voltage, the driving time or the driving waveform applied to a piezoelectric actuator of a head, a volume of ink droplets discharged from the head is directly controlled so as to vary the quantity of ink to be put on a recording medium, in other words, the area of dots.
This method exceeds in displaying gradation by discharge in a dot unit at each printing cycle, because by properly controlling the driving voltage, the driving waveform, or the time of the drive voltage to be applied to a piezoelectric actuator, a quantity of ink to be sucked into the ink chamber or a quantity of ink to be discharged can be controlled.
However, when the piezoelectric actuator is driven in this method, the amplitude, phase, attenuation time, etc. of the free vibrations generated in the piezoelectric actuator and the ink inside the ink chamber are changed due to the variation of the quantity of ink sucked into the ink chamber or the quantity of discharged ink. Accordingly, a position of a meniscus which is a face to discharge ink from is not stabilized, which accompanies a problem of dispersions of a diameter and discharge speed of ink droplets in the subsequent discharging motion, leading to an unstable discharging motion.
Furthermore, to vary the driving voltage, the driving waveform or the driving time applied to all of the piezoelectric actuators for each density that changes sequentially, it is necessary to prepare, for all of the piezoelectric actuators, respective circuits that generate the driving voltage, the driving waveform and the driving time, and drive the each driving circuit individually for each of the actuators, by which the entire driving circuits and control software become complicated, thus resulting in a difficulty to embody these.
The present invention has been made to solve the foregoing various problems of a conventional inkjet head printer and it is therefore an object of the invention to provide a circuit and a method for driving an ink-jet head that controls free vibrations of the piezoelectric actuator so as to discharge ink droplets of a stable quality with a virtually constant discharge time in a constant speed regardless of the size of ink droplets without deteriorating the resolution of the printing image by gradation in density.